If you lost your appmanifest.acf, try this.

If you find a game(here we take portal 2 as example) you installed appeared as uninstalled,but the game (portal 2) files are still in you computer.

Follow these steps:

Locate this folder:Steam\SteamApps

You should find several acf files named like this :appmanifest_*****.acf

Open the game(portal 2) store page, for example: http://store.steampowered.com/app/620/

and here "620" is the number you need.Try to find the appmanifest_620.acf,if it does not exist,create one file with that name(if you do not know the method,google it). PS:Control panel-->Folder options-->View-->Hide extensions to know files type, uncheck it. Create a new txt file, name its full name as " XXXXX.acf", not " XXXX.acf.txt", done.

open the appmanifest_620.acf ( if it is not portal 2 , replace 620 with your game number.)

it may looks like this:

Publicado originalmente por author:

"AppState"
{
"appid" "620"
"Universe" "1"
"name" "Portal 2"
"StateFlags" "66"
"installdir" "Portal 2"
"LastUpdated" "0"
"UpdateResult" "0"
"SizeOnDisk" "0"
"buildid" "0"
"LastOwner" "0"
"BytesToDownload" "0"
"BytesDownloaded" "0"
"AutoUpdateBehavior" "0"
"AllowOtherDownloadsWhileRunning" "0"
"UserConfig"
{
}
"MountedDepots"
{
}
}

Added in 2019/01/08, the portal 2 acf file here as I posted below:

Publicado originalmente por author:

"AppState"
{
"appid" "620"
"Universe" "1"
"name" "Portal 2"
"StateFlags" "4"
"installdir" "Portal 2"
"LastUpdated" "1546915851"
"UpdateResult" "0"
"SizeOnDisk" "12772699651"
"buildid" "3261795"
"LastOwner" "76561198124511924"
"BytesToDownload" "0"
"BytesDownloaded" "0"
"AutoUpdateBehavior" "0"
"AllowOtherDownloadsWhileRunning" "0"
"ScheduledAutoUpdate" "0"
"StagingFolder" "1"
"UserConfig"
{
"language" "english"
}
"InstalledDepots"
{
"624"
{
"manifest" "6415342253277896102"
}
"622"
{
"manifest" "2180841775043336221"
}
"621"
{
"manifest" "5315625579909971887"
}
"323180"
{
"manifest" "4821416258997072443"
"dlcappid" "323180"
}
}
"MountedDepots"
{
"624" "6415342253277896102"
"622" "2180841775043336221"
"621" "5315625579909971887"
"323180" "4821416258997072443"
}
}

If it looks like this( steam maybe update in the future,so I suggest you backup your file and try),change
"BytesDownloaded" "0" to
"BytesDownloaded" "12715331543", this number you can take randomly, then save it.

PS: don't forget to change the "name", "installdir" and "appid", also I don't know whether to change "Universe" and "StateFlags" or not, it would be best to search the formation of your game acf file from Google in case your acf file doesn't exist. Because if you open more acf files, they are different from each other in details.

Restart the steam,and install the game(portal 2) in your steam library,now it should begin to verify your game files instead of downloading it.

----------------------------------------------------------------------------------------------------------

It may take some time, that depends on your computer.

About this guide

When I happened to lost 1 game in the library of Steam, and I found the game folders and files still there, I think there should be a way to tell Steam:" the files are here, plz check out".

There are so many games in Steam, user may downloaded 100G file on their disk, it may take hours to check all the file to be correct, so it is impossible for Steam to check all the files in the starting( you can do that with option "verify integrity of game files" in the properties) , or we will wait Steam to start more than minutes every time when we start it. Actually when we start it, Steam knows which game is outdated and update it, so it does check something.

Searching in Google, not hard to find the information of acf file, but the game updated many times, I didn't find the latest one. Still I didn't find another friend who had the game and online to help me get the right file at that time.

So I focused on how to let Steam check the files. The acf file is easy to read,
"BytesDownloaded" "0"
"AutoUpdateBehavior" "0"
that tells Steam nothing downloaded, and the behavior make Steam downloading this game automatically.The best would be change it to the right figure which I have not. Still, I think if I change "downloaded" to none zero, Steam should verify the files to confirm the data have been downloaded. It worked at that time.

Still Steam may change this one day, so feel free if it doesn't work for you.

[NiGHTsC]

It's much simpler for me.
I have game files still in \common\ and non acf, so after I start steam, I just tell Steam to Install the game,
then it asks me all those same questions when you first install a game, like "Check to creat shortcut" "Disk space required XXX, Disk space available" etc etc...,
after I click Next it just start to verify my game files under \common\ , then it verify again under Downloads to see if I need update,
that's all.

[The Great Journey]

@Nexeir Right click your game in your library and select 'properties'. In the 'updates' tab on the side, you should see "App ID: xxx". The "xxx" will be the number you need.

[Nexeir]

where did the number come from?

[RİMURU TEMPEST]

594650

[Dell Conagher]

please help i need appmanifest_287390

[Hollowgunn]

downloaded an empty folder with the game name :/

[snarkqueen]

This worked. Thank you so much! I wasn't looking forward to redownloading a large game!

[GigaChad´s Grandson]

I dont know how but after trying this method for 4 hours I did something right and it worked, so thanks mate I guess
I think I have changed the stateflag to 4

[kouvola_]

i dont get how it should work

[tuxutku]

Edit: On linux, if you're playing the windows version of a linux game, don't forget to first enable "force the use of a specific Steam Play compatibility tool" under game properties. It doesn't have to be the exact version in your prefix.
Also I found "finding pre-existing files" a bit buggy. You may try couple times if it refuses to see the files.